This invention relates to novel carboxylic acid ester compounds and liquid crystal compounds, liquid crystal compositions and liquid crystal elements using these compounds, the method of producing said liquid crystal elements, and their uses.
Conventionally, a CRT device has been most widely used as a display device for office automation (OA) equipments. Recently, in the field of OA equipments having such display device, a small-sized and light-weighted equipment and an enlarged and thinned scope of display device have been increasingly desired. To meet the respective uses and desires, various new display devices have been developed, instead of the conventionally used CRT device. Such display devices include, for example, liquid crystal display, plasma display, Light Emitting Diode display, Electro Luminesence display, and ECD display.
Among these display devices, the liquid crystal display is a device for actualizing an electric signal on a scope by affording the electric signal to a switching element using a liquid crystal compound, changing the state of the liquid crystal compound in the switching element correspondingly to the electric signal, thereby controlling the light permeability. This liquid crystal device has been already put into practical use not only as the liquid crystal display for OA equipments but also as a display device for digital watch and portable game machine. It also comes to be used as a display device for dynamic image such as a small-sized television.
The display device using a liquid crystal compound can be driven in a twist nematic (TN) mode. This TN mode is a system of utilizing the dielectric anisotropy of the molecule in the nematic phase of the liquid crystal compound for displaying, and the display device is driven by the energy proportional to the square of an externally applied electric field (f.alpha.E.sup.2).
However, as a change of the molecular position of the liquid crystal compound in the element is required to change a displayed picture image when this system is adapted, it has problems of an extension of the driving time, and thus an increase in the voltage required to change the molecular position of the liquid crystal compound, namely an increase in the consumed electric power. Further, as such switching element is not extremely excellent in switching threshold characteristic, a leakage voltage is applied to the extent of the non-displayed picture image part when a high-speed change of the molecular position for switching operation is tried, often resulting in a remakable reduction in contrast of the display device.
For the reasons mentioned above, the display system by the conventional TN mode is not perfectly proper display system, particularly, as a display device for large-sized image plane or a display device for dynamic image such as small-sized digital television.
In the meantime, a display device using a super twist nematic (STN) mode in which the switching threshold characteristic of the TN mode is improved has been also used. As the switching threshold characteristic is improved by using the STN mode, the contrast of the display is improved.
However, this method is not different from the TN mode in the utilization of dielectric anisotropy, and thus the similar tendency as the display device using the TN mode is shown when used as the display device for large image plane or the display device for dynamic image such as small-sized digital television, because of this long switching time.
Contrary to this, in 1975, R. B. Meyer et al, found that an organic compound synthesized by them showed a ferroelectric property. In 1980, further, N. A. Clark et al. suggested a possibility of using an element in which the ferroelectric liquid crystal compound as described above is filled in a cell having a small gap as an optical switching element, i.e., a display device (Japanese Patent Application No. 56-107216).
The switching element using the above ferroelectric liquid crystal compound can be actuated as the switching element only by changing the orientation direction of the molecule of the liquid crystal product, different from the switching elements utilizing the TN mode or STN mode, so that the switching time is extremely shortened. Further, as the value of Ps.times.E which is given by the spontaneous polarization (Ps) and the electric field strength (E) possessed by the ferroelectric liquid crystal compound is an effective energy strength for changing the orientation direction of the molecule in the liquid crystal compound, the consumed electric power is extremely reduced. This ferroelectric liquid crystal compound is extremely excellent in switching threshold characteristic, and particularly suitably used for dynamic image display devices, as it has two stable states or bistability, depending on the direction of the applied electric field.
When used in the optical switching element, the ferroelectric liquid crystal compound is required to have various characteristics such as an operating temperature range around or less than ordinary temperature, a broad operating temperature width, a high(rapid) switching speed, and a switching threshold value voltage within a proper range. Particularly, the operating temperature range is an especially important factor for the practical use of ferroelectric liquid crystal compounds.
However, the conventionally known ferroelectric liquid crystal compounds generally have high operating temperatures, and the operating temperature width and other characteristics are not sufficient even if they are ferroelectric liquid crystal compounds operating at around room temperature, as described in, for example, a paper by R. B. Meyer et al. [J. de Phys., Vol. 36, p. L-69, 1975]; a paper by Masaaki Taguchi and Takamasa Harada, [the Preliminary Manuscripts for the 11th Liquid Crystal Discussion, p. 168, 1985], and thus those practically satisfied as ferroelectric liquid crystal compounds have not been obtained yet.
Further, various liquid crystal compounds have been known. In Japanese Patent Laid Open Nos. 62-10045, 62-135449 and 63-233932, carboxylic acid ester compounds having a similar structure of molecule to the present invention have been disclosed and such compounds showing liquid crystal properties are also discussed. However, the compounds in the above disclosures are compounds which contain an asymmetric carbon atom bonded to a methyl group. These compounds should be distinguished from the present invention. In addition, when used as a liquid crystal composition, these compounds have disadvatages because of having a weak contrast. For example, when a liquid crystal compound of the following formula (C) where an asymmetric carbon element and a methyl group instead of an ethyl group are bonded is used in a liquid crystal element, a tendency to show a weak contrast is obtained. Moreover, a liquid crystal compound having propyl or more highly alkyl group instead of ethyl groups is not only hard to synthesis themselves, but also when used as a liquid crystal element, an asymmetric carbon atom becomes high in rigidity and a tendency of reducing the liquid crystal properties is seen. ##STR2##
Furthermore, a carboxylic acid ester compound comprising the bonding of an asymmetric carbon atom to a trifluoromethyl group is disclosed in Japanese Patent Laid Open No. 1-139551. However, this compound has a different main chain of molecule from the present invention.